Increasing demand for wireless connectivity has brought a continued surge in wireless traffic data and congestion in wireless environments. Wireless LAN (WLAN) is widely employed for wireless data transfer and is available on nearly any kind of wireless devices, which results in more and more traffic over-the-air in congested WLAN bands.
WLAN is especially but not exclusively used herein as synonymous or comprising the IEEE 802.11 set of standard specifications. IEEE 802.11 defines amongst others a set of Media Access Control (MAC) and Physical Layer (PHY) specifications for implementing WLAN computer communications in the 2.4, 3.6, 5 and 60 GHz frequency bands. The standards provide the basis for wireless network products such as those using the Wi-Fi brand.
WLAN is based on “Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA), operating in the Data Link Layer, which is a network multiple access method for data transmissions. According to ‘Carrier Sense’, prior to transmitting a node listens to wireless signals to determine whether or not another node is currently transmitting on the channel. When a node transmits, it transmits all the packet data to be transmitted in its entirety.
Nodes attempt to avoid collisions by transmitting only when the channel is sensed to be idle. According to the ‘Collision Avoidance’ approach of CSMA/CA, the channel is intended to be about equally divided among all transmitting nodes within the collision domain. If another node is detected transmitting, the node waits for a time span before attempting again to access the channel.
IEEE 802.11 devices use a ‘Distributed Coordination Function’ (DCF). DCF is defined in subclause 9.2 of the IEEE 802.11 standard and is the de facto default setting for Wi-Fi hardware, for example. DCF requires a station wishing to transmit to listen for the channel status for a predefined fixed time span, the DCF Interframe Space Interval (DIFS). If the channel is found busy during the DIFS interval, the station defers its transmission. In order to avoid collisions between multiple stations contending for the medium, DCF further specifies a random backoff time or ‘backoff’, which forces a station to defer its access to the channel for a further, extra period. The length of the backoff period is determined by a random number between zero and a given maximum number defining multiples of a predefined fixed slot time.
The maximum number multiplied by the slot time defines the ‘contention window’, from within which the station randomly selects its (next) access attempt. The contention window size increases with the retransmission attempts. DCF further implements an ‘exponential backoff’ scheme, according to which the window size is multiplicatively increased. Typically a binary scheme is applied, i.e. the maximum number of time slots may increase from a minimum contention window size (CW_min) of, e.g., 7 time slots for the initial transmission attempt to 15, 31, 63, 127, 255, . . . for the first, second, third, fourth, fifth, . . . retransmission attempts.
DCF includes a positive acknowledge scheme in the MAC layer, which means that if a frame is successfully received by the destination it is addressed to, the destination needs to send an acknowledgement frame (ACK frame) to notify the source of the successful reception.
The backoff mechanism significantly reduces the number of collisions and retransmissions in case of increasing number of devices. However, from the point of view of one of these devices, the random numbers to be used for (re)transmission attempts impact the performance. The larger the random numbers, the longer the backoff times, and the higher the chances of avoiding collisions and allowing other devices to access the channel; however this comes at the cost of decreasing effective data throughput. The smaller the random numbers, the shorter the backoff times and the smaller the intervals between consecutive data transmissions of the device, i.e. the higher the device's data throughput and performance.
There is a high demand for data throughput, illustrated for example with the emergence of new technologies like IEEE 802.11ac which is designed for very high throughput.
The IEEE 802.11 standards only provide for technical specifications but do not prescribe how to implement features. WLAN devices could be tested in order to, for example, characterize them according to their performance.
Bianchi et al., “Experimental assessment of the backoff behavior of commercial IEEE 802.11b network cards”, IEEE INFOCOM 2007, proceeding of the 26th IEEE International Conference on Computer Communications, 6-12 May 2007, p. 1181-1189, discloses an experimental characterization of the backoff operations of a plurality of network interface cards. Low-level backoff distribution measurements are taken. Non-standard backoff behavior is detected, e.g. in terms of minimum contention window size. Further, some of the cards seem to suffer from implementation limits in either the card hardware/firmware and/or the software drivers.
It therefore turns out from Bianchi et al. that WLAN devices may show different performance and may especially not perform as expected in terms of, e.g., backoff operation. Therefore there is in fact a need for efficient techniques for testing performance compliance of WLAN devices.
Backoff time is randomly selected within the contention window each time, which requires a comprehensive technique for backoff time measurement and analysis in order to characterize the backoff time performance across multiple WLAN devices. Bianchi et al. performed experiments and an in-depth comparative analysis of multiple network cards and clearly showed the performance differences thereof. However, the document does not provide a framework which would allow developing test scenarios which could efficiently be applied for fully and reliably characterizing and comparing a plurality of WLAN cards in terms of their network performance.
It is one object to develop a technique which could routinely and efficiently be applied for testing WLAN devices and which enables fully and reliably characterizing and comparing a plurality of WLAN devices in terms of their network performance.